1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, which has a stackless ADU and can execute black-and-white or color 2-sided printing.
2. Description of the Related Art
In the prior art, in general, when 2-sided printing is performed using a stackless ADU, alternate circulation of paper sheets is executed if the paper sheets are of short size. Thereby, a decrease in performance is minimized. In the alternate circulation, a path length that can retain two or more short-size paper sheets is secured in the ADU. Two sheets are first fed into the ADU, and then alternate printing is performed between the ADU and the sheet-supply side.
In the 2-sided printing, when color printing requires four rotations of transfer (e.g. revolver-type 4-color transfer), a subsequent paper sheet stays in the ADU for a long time in the alternate circulation.
The upper part of the ADU, however, is located just above the heat roller of a fixing device. Moisture in the paper sheet that stays in the ADU is lost due to the heat of the heat roller. When printing is effected on the paper sheet that comes from the ADU, good secondary transfer of toner could not be performed and toner would disperse.